Solenoid's just turn the co2 on/off, usually done with a timer. Many, including me, have run co2 24/7. You are basically wasting co2 when you run it at night, didn't have a solenoid so I didn't really have a choice. Plant's use o2 at night and need co2 during lights on to photosynthesize. The main thing you need for a co2 setup is a tank, regulator, needlevalve, and diffusor/reactor. A solenoid is something that is not absolutely needed but helpful. Also, get a decent needle valve. The fabco nv-55 is bar none, the best bang for the buck. You can find it for a little over $20. There are better one's out there, but the fabco will do the job just fine. As for the milwaukee regulators, I've read that they are hit/miss. Some love them and some just hate them. The main problem is that the needle valve may not be very consistent and the solenoid sometimes sticks. I've never owned one of those regulators so I am just going off of what I had read when I was shopping around for a regulator. I ended up getting my regulator here. They can build you a solid regulator that you will have no problems with and will last a lifetime. You also check these site's out too: rex and gla. Not the cheapest but solid built regulators.